[Q] How to wipe OS and reinstall

Hello, i have a Samsung S III I9300, on which after a lot of custom ROMs, i have installed a clean original 4.1.2 firmware the UK version or something, and now i just flashed the 4.3 Orange Romania, which is my provider. The problem is that when i try to let's say search for updates it tells me that i have a custom "bla bla" (can't really remember what it said), and on kies it shows this http: //s29.postimg. org/pacbqv0h3/image.jpg which doesn't look ok to me.

So my question is, how can i wipe it all and install my fresh 4.3 Orange Romania firmware, so that i will get all of those "new phone, straight out the box" settings, AND make it so that it will appear like the firmware is perfectly fine and official, the kind that could get updates and whatnot lol.

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